Environmental pollution with hydrocarbons poses a major concern. Crude oil is a major sea pollutant, and petroleum products, such as gasoline and diesel fuel and fuel oils, are the most frequent organic pollutants of soils and ground waters. In the drilling of oil and gas wells, oil-based drilling fluid is required in most of the challenging drilling situations, and the spent oil-coated drill cuttings cannot typically be discharged from the drilling rig for environmental reasons. A rapid biodegradation of oil on such cuttings could render oil-based drilling fluids as environmentally acceptable as water-based drilling fluids.
There are two primary types of drilling fluids: (i) water based drilling fluids (WBF); and (ii) non-aqueous drilling fluids (NADFs). WBFs comprise water mixed with bentonite clay and barite to control mud density and thus, hydrostatic head. Other substances can be added to affect one or more desired drilling properties. NADFs are either based on mineral oil, diesel, or synthetic base fluid. NADFs are typically water in oil (invert) emulsions. In rare cases, such as with coring fluids, 100% oil-based drilling fluids have been used. NADFs are generally preferred over water-based fluids for their ability to provide superior borehole stability, lubricity, rate of penetration, stuck pipe prevention, chemical stability, and corrosion protection.
In contrast to WBFs and WBF-coated cuttings that can typically be discharged into the environment, in many areas regulatory standards do not allow the discharge of NADFs, or drill cuttings coated with NADF into the environment. If NADF-coated cuttings are not permitted to be discharged into the environment, then the cuttings must either be reinjected, hauled to shore, thermally treated to remove base fluid, or land farmed. In some regions, drill cuttings coated with NADF can be discharged into the environment if the base fluid and/or whole mud is approved for discharge. In many cases, cutting dryers are used to remove most of the NADF from the cuttings prior to discharge.
The inability to discharge technically superior NADF and NADF cuttings into the environment presents a huge problem for the oil and gas industry. In many drilling situations, NADFs must be used in order to economically drill the well. This is particularly true with high angle wells, horizontal wells, high pressure high temperature wells, deepwater wells, slimhole wells, and wells drilled into water-sensitive formations.
Many technologies have been developed to deal with the problem of NADF disposal. However, each of these systems has limitations. Cuttings drying is expensive and can only achieve a reduction in oil on cuttings down to 3-4% by weight. The injection of cuttings containing NADF has limitation due to the equipment requirement to capture the cuttings, slurrify them and pump them down an annulus, the lack of available annuli, and the poor understanding of the fracture process involved. Hauling of cuttings containing NADF is expensive and results in non-water quality environmental impacts, including air pollution from transportation, energy use during transportation, and disposal site factors. Landfarming of cuttings requires large areas of land, is a slow process and creates environmental concerns due to the potential for leaching and runoff. Thermal processing of cuttings is expensive, requires a large footprint, and creates safety concerns due to the high temperatures involved.
Thus, methodologies that make the drill cuttings more environmentally acceptable would be valuable.